Conventionally, a vehicle is equipped with a fuse function to disconnect a load from a power source when the overcurrent or the like occurs. Recently, additionally, there is growing demand for miniaturize a junction box (J/B) or a relay box (R/B) in a vehicle equipped with the fuse function. Correspondingly, there is a glowing tendency of replacing a mechanical fuse with a semiconductor switching element to provide a vehicle with the fuse function.
As the technique of realizing the fuse function by a semiconductor switching element, for instance, there has been conventionally proposed a technique of controlling a semiconductor switching element for switching on and off the power supply to a load by a vehicular ECU (Electronic Control Unit) connected to the semiconductor switching element.
According to this proposal, a microprocessor in an ECU receives, from the other ECU, a switch signal in association with switching-on/off operation of a switch connected to the other ECU. Then, based on the received switch signal, the power supply to the load is switched on and off by the semiconductor switching element, in association with the switching-on/off operation of the switch.
Thus, the microprocessor judges whether or not there is an abnormal fuse state where the power supply to the load should be restricted, from a load current (current flowing through the load) detected by a sensing circuit in the ECU. Then, if it is judged that there exists such an abnormal fuse state, the microprocessor controls to turn on/off the semiconductor switching element in preference to a switch signal brought from the other ECU, so that the power supply to the load is suppressed.
It is noted that the ECU includes an analogue circuit for turning on/off the semiconductor switching element in association with the switch signal transmitted from the other ECU, separately. The function of this analogue circuit is invalidated during the normal functioning of the microprocessor to output an output signal periodically.
Therefore, when the microprocessor becomes impossible to control to turn on/off the semiconductor switching element due to runaway or the like, it is possible to turn on/off the semiconductor switching element in accordance with the content of the switch signal by the analogue circuit which is validated with the shutdown of the output signal from the microprocessor (see JP 5037414 B).